Fiber optic communication networks and the like are experiencing rapidly increasing capacity growth. This capacity growth is reflected in individual channel data rates inexorably scaling from 10 Gbps, to 40 Gbps, to 100 Gbps, to 1000 Gbps channels, and so on. The capacity growth is also reflected in increasing total channel counts carried within an optical fiber, for example.
The ever growing demand for increased bandwidth and channel capacity is being met by broadband polarization division multiplexed (PDM) coherent systems and the like utilizing multi lever amplitude/phase modulation formats, for example. In such systems, the essential tasks of pre-conditioning the signal at the transmitter and signal processing after coherent detection are carried out by extremely fast digital signal processing (DSP) integrated circuit (IC) chips. The tendency towards dramatic growth of the DSP chips' size, complexity, price, and power consumption and dissipation with ever increasing processing speeds is well researched and thoroughly documented.
A significant portion of the DSP algorithms and computing power is devoted to the solution of two crucial and computationally intense tasks: polarization de-multiplexing of received signals and laser frequency offset recovery and phase noise cancellation. The former involves the estimation, calculation, and application of the elements of the inverse Jones matrix of the optical fiber link for the recovery of the originally linearly polarized signals, while the later involves the determination of the fast spinning phasor generated by the beating of the signal carrier and local oscillator (LO) lasers and its utilization for phase locking of the coherently detected signals and rectifying them for the following steps of the data recovery processes. These computationally intense algorithms are accountable for the majority of the DSP chips' real estate and power usage.
As an alternative, pilot assisted techniques designed to perform the above tasks were conceived at the early onset of coherent fiber optic communication system design, long before the DSP era. The un-modulated, but either frequency shifted or orthogonally polarized, portion of the signal carrier laser light was launched into the optical fiber along with the information carrying signal and served as a built in phase noise reference at the receiver. Down mixing of the signal with the pilot at the receiver (i.e. multiplying the signal W by the complex conjugate pilot P*: W×P*) canceled their common noisy phase factors originating from the random phase walk due to finite linewidth of the signal carrier and LO lasers, as well as de-multiplexing the polarizations.
These pilot based signal processing techniques re-surfaced recently, allowing not only for much simpler ways of cancelling laser frequency offset and phase noise, but also for compensating the nonlinear phase distortion, greatly reducing or even eliminating the need for fast, expensive, and power hungry digital electronic signal processing.
However, all of the existing pilot assisted techniques suffer from one common drawback: the noise accompanying the pilot, mostly amplified spontaneous emission (ASE) noise generated in optical amplifiers, significantly compromises the pilot's ability to perform its functions, resulting in a pilot related signal quality (Q) penalty. Special measures are required to reduce or even eliminate the pilot noise related Q penalty.